The present invention generally relates to fluid control devices and, more particularly, to micromachined synthetic jet actuators for controlling fluid flows though creation of a synthetic jet stream to interact with the fluid flow.
The ability to manipulate and control the evolution of shear flows has tremendous potential for influencing system performance in diverse technological applications, including: mixing and combustion processes, lift and drag of aerodynamic surfaces, and thrust management. That these flows are dominated by the dynamics of a hierarchy of vortical structures, evolving as a result of inherent hydrodynamic instabilities (e.g., Ho and Huerre, 1984), suggests control strategies based on manipulation of these instabilities by the introduction of small disturbances at the flow boundary. A given shear flow is typically extremely receptive to disturbances within a limited frequency band and, as a result, these disturbances are rapidly amplified and can lead to substantial modification of the base flow and the performance of the system in which it is employed.
There is no question that suitable actuators having fast dynamic response and relatively low power consumption are the foundation of any scheme for the manipulation and control of shear flows. Most frequently, actuators have had mechanically moving, parts which come in direct contact with the flow [e.g., vibrating ribbons (Schubauer and Skramstad J. Aero Sci. 14 1947), movable flaps (Oster and Wygnanski, 1982), or electromagnetic elements (Betzig AIAA, 1981)]. This class of direct-contact actuators also includes piezoelectric actuators, the effectiveness of which has been demonstrated in flat plate boundary layers (Wehrmann 1967, and Jacobson and Reynolds Stan. U. TF-64 1995), wakes (Wehrmann Phys. Fl. 8 1965, 1967, and Berger Phys. Fl. S191 1967), and jets (Wiltse and Glezer 1993). Actuation can also be effected indirectly (and, in principle, remotely) either through pressure fluctuations [e.g., acoustic excitation (Crow and Champagne JFM 48 1971)] or body forces [e.g., heating (Liepmann et al. 1982, Corke and Mangano JFM 209 1989, Nygaard and Glezer 1991), or electromagnetically (Brown and Nosenchuck, AIAA 1995)].
Flow control strategies that are accomplished without direct contact between the actuator and the embedding flow are extremely attractive because the actuators can be conformally and nonintrusively mounted on or below the flow boundary (and thus can be better protected than conventional mechanical actuators). However, unless these actuators can be placed near points of receptivity within the flow, their effectiveness degrades substantially with decreasing power input. This shortcoming can be overcome by using fluidic actuators where control is effected intrusively using flow injection (jets) or suction at the boundary. Although these actuators are inherently intrusive, they share most of the attributes of indirect actuators in that they can be placed within the flow boundary and require only an orifice to communicate with the external flow. Fluidic actuators that perform a variety of xe2x80x9canalogxe2x80x9d (e.g., proportional fluidic amplifier) and xe2x80x9cdigitalxe2x80x9d (e.g., flip-flop) throttling and control functions without moving mechanical parts by using control jets to affect a primary jet within an enclosed cavity have been studied since the late 1950""s (Joyce, HDL-SR 1983). Some of these concepts have also been used in open flow systems. Viets (AIAA J. 13 1975) induced spontaneous oscillations in a free rectangular jet by exploiting the concept of a flip-flop actuator and more recently, Raman and Cornelius (AIAA J. 33 1995) used two such jets to impose time harmonic oscillations in a larger jet by direct impingement.
More recently, a number of workers have recognized the potential for MEMS (micro electro mechanical systems) actuators in flow control applications for large scale systems and have exploited these devices in a variety of configurations. One of a number of examples of work in this area is that of Ho and his co-investigators (e.g., Liu, Tsao, Tai, and Ho, 1994) who have used MEMS versions of xe2x80x98flapsxe2x80x99 to effect flow control. These investigators have opted to modify the distribution of streamwise vorticity on a delta wing and thus the aerodynamic rolling moment about the longitudinal axis of the aircraft.
It was discovered at least as early as 1950 that if one uses a chamber bounded on one end by an acoustic wave generating device and bounded on the other end by a rigid wall with a small orifice, that when acoustic waves are emitted at high enough frequency and amplitude from the generator, a jet of air that emanates from the orifice outward from the chamber can be produced. See, for example, Ingard and Labate, Acoustic Circulation Effects and the Nonlinear Impedance of Orifices, The Journal of the Acoustical Society of America, March, 1950. The jet is comprised of a train of vortical air puffs that are formed at the orifice at the generator""s frequency.
The concern of scientists at that time was only with the relationship between the impedance of the orifice and the xe2x80x9ccirculationxe2x80x9d (i.e., the vortical puffs, or vortex rings) created at the orifice. There was no suggestion to combine or operate the apparatus with another fluid stream in order to modify the flow of that stream (e.g., its direction). Furthermore, there was no suggestion that following the ejection of each vortical puff, a momentary air stream of xe2x80x9cmake upxe2x80x9d air of equal mass is drawn back into the chamber and that, as a result, the jet is effectively synthesized from the air outside of the chamber and the net mass flux out of the chamber is zero. There was also no suggestion that such an apparatus could be used in such a way as to create a fluid flow within a bounded (or sealed) volume.
Such uses and combinations were not only not suggested at that time, but also have not been suggested by any of the ensuing work in the art. So, even though a crude synthetic jet was known to exist, applications to common problems associated with other fluid flows or with lack of fluid flow in bounded volumes were not even imagined, much less suggested. Evidence of this is the persistence of certain problems in various fields which have yet to be solved effectively.
Vectoring of a Fluid Flow
Until now, the direction of a fluid jet has chiefly been controlled by mechanical apparatus which protrude into a jet flow and deflect it in a desired direction. For example, aircraft engines often use mechanical protrusions disposed in jet exhaust in order to vector the fluid flow out of the exhaust nozzle. These mechanical protrusions used to vector flow usually require complex and powerful actuators to move them. Such machinery often exceeds space constraints and often has a prohibitively high weight. Furthermore, in cases like that of jet exhaust, the mechanism protruding into the flow must withstand very high temperatures. In addition, large power inputs are generally required in order to intrude into the flow and change its direction. For all these reasons, it would be more desirable to vector the flow with little or no direct intrusion into the flow. As a result, several less intrusive means have been developed.
Jet vectoring can be achieved without active actuation using the coanda effect, or the attachment of a jet to a curved (solid) surface which is an extension one of the nozzle walls (Newman, B. G. xe2x80x9cThe Deflexion of Plane Jets by Adjacent Boundaries-Coanda Effect,xe2x80x9d Boundary Layer and Flow Control v. 1, 1961 edited by Lachmann, G. V. pp. 232-265.). However, for a given jet momentum, the effect is apparently limited by the characteristic radius of the curved surface. The effectiveness of a coanda surface can be enhanced using a counter current flow between an external coanda surface and a primary jet. Such a system has been used to effect thrust vectoring in low-speed and high-speed jets by Strykowski et al. (Strykowski, P. J, Krothapalli, A., and Forliti D. J. xe2x80x9cCounterflow Thrust Vectoring of Supersonic Jets,xe2x80x9d AIAA Paper No. 96-0115, AIAA 34th Aerospace Sciences Meeting, Reno, Nev., 1996.).
The performance of coanda-like surfaces for deflection of jets can be further improved by exploiting inherent instabilities at the edges of the jet flow when it is separated from the surface. It has been known for a number of years that substantial modification of shear flows can result from the introduction of small perturbations at the boundaries of the shear flow. This modification occurs because the shear flow is typically hydrodynamically unstable to these perturbations. Such instability is what leads to the perturbations"" rapid amplification and resultant relatively large effect on the flow. This approach has been used in attempts to control separating flows near solid surfaces. The flow typically separates in the form of a free shear layer and it has been shown that the application of relatively small disturbances near the point of separation can lead to enhanced entrainment of ambient fluid into the layer. Because a solid surface substantially restricts entrainment of ambient fluid, the separated flow moves closer to the surface and ultimately can reattach to the surface. This effect has been used as a means of vectoring jets near solid surfaces. See e.g., Koch (Koch, C. R. xe2x80x9cClosed Loop Control of a Round Jet/Diffuser in Transitory Stall,xe2x80x9d PhD. Thesis, Stanford University, 1990) (using wall jets along in a circular diffuser to effect partial attachment and thus vectoring of a primary round jet).
Similar to mechanical deflectors, technologies that rely on coanda surfaces are limited because of the size and weight of the additional hardware. Clearly, a coanda collar in aerospace applications must be carried along at all times whether or not it is being used.
Fluidic technology based on jet-jet interaction has also been used for flow vectoring or producing oscillations of free jets. Fluidic actuators employing control jets to affect a primary jet of the same fluid within an enclosure that allows for inflow and outflow have been studied since the late 1950""s. These actuators perform a variety of xe2x80x9canalogxe2x80x9d (e.g., proportional fluidic amplifier) and xe2x80x9cdigitalxe2x80x9d (e.g., flip-flop) throttling and control functions in flow systems without moving mechanical parts (Joyce, 1983). In the xe2x80x9canalogxe2x80x9d actuator, the volume flow rate fraction of two opposite control jets leads to a proportional change in the volume flow rate of the primary stream out of two corresponding output ports. The xe2x80x9cdigitalxe2x80x9d actuator is a bistable flow device in which the control jets and Coanda effect are used to direct the primary stream into one of two output ports.
These approaches have also been employed in free jets. Viets (1975) induced spontaneous oscillations in a free rectangular jet by exploiting the concept of a xe2x80x9cflipflopxe2x80x9d actuator. More recently, Raman and Cornelius (1995) used two such jets to impose time harmonic oscillations in a larger jet by direct impingement. The control jets were placed on opposite sides of the primary jet and could be operated in phase or out of phase with each other.
Use of a fluidic jet to vector another flow, while known for years, has been used with limited success. In particular, the only way known to vector a jet with another jet (dubbed a xe2x80x9ccontrol jetxe2x80x9d) of the same fluid was to align the control jet so that it impinges directly on the primary jet. Obviously, this involved injection of mass into the flow and has not been deemed very effective at vectoring the primary flow because it relies on direct momentum transfer between the jets for altering the direction of the primary jet. Direct momentum transfer is not economical in general and undesirable when the available power is limited (such as on board an aircraft). Furthermore, such control hardware is difficult and expensive to install because of the complex plumbing necessary to supply the control jet with fluid to operate.
Modification of Fluid Flows about Aerodynamic Surfaces
The capability to alter the aerodynamic performance of a given airframe by altering its shape (e.g., the xe2x80x9ccamberxe2x80x9d of an airfoil) during various phases of the flight can lead to significant extension of the airframe""s operating envelope. Geometric modification of lifting surfaces has so far been accomplished by using mechanical fla0s and slats. However, because of the complex control system required, such devices are expensive to manufacture, install and maintain. Furthermore, flap systems not only increase the weight of the airframe, but also require considerable interior storage space that could be used for cargo, and additional ancillary hardware (e.g., hydraulic pumps, piping, etc.). In some applications, the weight penalty imposed by the flaps may more than offset their usefulness.
In addition to the use of mechanical flaps, there has been considerable effort to enhance the aerodynamic performance of lifting surfaces by delaying flow separation and thus the loss of lift and increase in drag. Conventional methods for such flow control have primarily focused on delay of separation or inducement of reattachment by introducing small disturbances into the upstream wall boundary layer. Excitation methods have included external and internal acoustic excitation (Huang, Maestrello and Bryant, Expt. Fl. 15 1987), vibrating flaps (e.g., Neuberger and Wygnanski, USAF A TR-88 1987) and unsteady bleeding or blowing (e.g., Sigurdson and Roshko, AIAA 1985, and Seifert, Bachar, Koss, Shepshelovich and Wygnanski, AIAA J. 31 1993). These methods have been used with varying degrees of success. The effectiveness largely depends on the receptivity of the boundary layer to excitation within a relatively narrow bandwidth.
Other efforts of designers to modify the flow about an aerodynamic surface have centered on injection of energy into the boundary layer of the flow in order to augment lift, reduce drag, delay turbulent onset, and/or delay flow separation. For example, the method disclosed by U.S. Pat. No. 4,802,642 to Mangiarotty involves the retardation of a flow""s transition to turbulence. However, this prior art does not and cannot change the effective aerodynamic shape of the airfoil. That is, the apparatus cannot change the direction of flow of the freestream fluid about the surface. Instead, the apparatus propagates acoustic excitation above the Tollmien-Schlichting frequency in an attempt to disrupt Tollmien-Schlichting waves as they begin to form and thereby delay the onset of turbulence. Although this method changes the drag characteristic of a lifting surface, the mean velocity field and thus apparent aerodynamic shape of the surface remain unchanged.
Such devices as slots and fluid jets have also been extensively employed to inject energy into the boundary layer in order to prevent flow separation. Recently, efforts have turned to the use of piezoelectric or other actuators to energize the boundary layer along an aerodynamic surface. See, e.g., U.S. Pat. No. 4,363,991 to Edleman. These techniques, which employ acoustic excitation, change the surface aerodynamic performance by suppressing the naturally occurring boundary layer separation. This method requires the flow state to be vulnerable to specific disturbance frequencies. Although effective at delaying flow separation, none of these devices alter the apparent aerodynamic shape or mean velocity field of a given aerodynamic surface. Even though the changes in lift and drag that are caused by separation can be somewhat restored, there is no effect before separation occurs and the locus of the stagnation points remain largely unchanged. Therefore, before the present invention, there was no way to alter the effective shape of an aerodynamic surface without the complexity, high expense, and weight penalty of mechanical flaps or slats.
Mixing of Fluids at the Small Scale Level
In a somewhat different field of study, the ability to effectively control the evolution of the shear layer between two streams of similar fluids (gas or liquid) may have great impact on the mixing between the two streams (e.g., mixing a hot exhaust plume with cold ambient air). The boundary between the two streams forms the turbulent flow region known as a xe2x80x9cshear layer.xe2x80x9d Hydrodynamic instabilities in this shear layer induce a hierarchy of vortical structures. Mixing between the two streams begins with the entrainment of irrotational fluid from each stream by the large-scale vortical structures. These vortical structures scale with geometric features of the flow boundary (e.g., nozzle diameter of a jet, vortex generators, etc.) and they are critical to the mixing process between the two streams by bringing together in close contact volumes of fluid from each stream in a process that is referred to as entrainment. Layers of entrained fluid are continuously stretched and folded at decreasing scales by vortical structures that evolve through the action of shear and localized instabilities induced by larger vortical structures. This process continues until the smallest vortical scales are attained and fluid viscosity balances the inertial forces. This smallest vortical scale is referred to as the Kolmogorov scale. Thus, a long-held notion in turbulence is that the smallest and largest turbulent motions are indirectly coupled through a cascade of energy from the largest to successively smaller scales until the Kolmogorov scale is reached and viscous diffusion can occur. Turbulent dissipation is the process by which (near the Kolmogorov scale) turbulent kinetic energy is converted into heat as small fluid particles are deformed.
Scalar mixing (of heat or species, for example) is similar, but not identical to momentum mixing. Analogous to the Kolmogorov scale, the Batchelor scale is the smallest spatial scale at which an isoscalar particle can exist before scalar gradients are smoothed by the action of molecular diffusion. If scalar diffusion occurs on a faster scale than momentum diffusion, the Kolmogorov energy cascade breaks xe2x80x9cpacketsxe2x80x9d of scalars down into scales small enough that molecular scalar diffusion can occur (at the Batchelor scale). In this case, the Batchelor scale is larger than the Kolmogorov scale and turbulent motions persist at scales where scalar gradients have been smoothed out by diffusion. If, on the other hand, scalar diffusion occurs on a slower scale than momentum diffusion, turbulent motions stop (at the Kolmogorov scale) before the scalar gradients are smoothed out. Final mixing only occurs after laminar straining further reduces the size of the scalar layers.
There is a substantial body of literature that demonstrates that mixing in shear flows can be influenced by manipulating the evolution of the large scale eddies (vortical structures) within the flow. Because the large-scale eddies result from inherent hydrodynamic instabilities of the flow, they can be manipulated using either passive or active devices.
As noted above, although the entrainment process in turbulent shear flows is effected by the large-scale eddies, molecular mixing ultimately takes place at the smallest scales which is induced by a hierarchy of eddies of decreasing scales that continuously evolve from the largest scale eddies. Because the base flows are normally unstable at the large scales (and thus receptive to either passive or active control inputs), the traditional approach to controlling mixing at the small-scale has been indirect. Previous attempts to control small-scale mixing employing both passive and active control strategies have relied on manipulation of global two-and three-dimensional instability modes of the base flow with the objective of controlling mixing through the modification of the ensuing vortical structures.
Passive control has primarily relied on (permanent) modification of the geometry of the flow boundary. For example, mixing of jet exhaust is often enhanced by corrugating the exhaust port of a jet. This corrugation creates the appearance of a number of lobes defined by raised and recessed curves which induce counter-rotating vortices, thus promoting mixing in the direction of the exhaust flow. Other passive devices for the promotion of mixing have included small tabs that act as vortex generators. The disadvantage of such mixing devices is that their geometry is fixed and thus their effectiveness cannot be adjusted for varying flow conditions.
Conventional active control strategies overcome this deficiency because the control input can be adjusted. For example, one prior disclosure describes the manipulation of large scale eddies in a plane shear layer between two uniform streams using a small oscillating flap. However, because this approach depends on the classical cascading mechanism to transfer control influence to the scales at which molecular mixing occurs, mixing at the smallest scales in fully turbulent flows is only weakly coupled 4o the control input. More importantly, mixing control of this nature relies on a priori knowledge of the flow instabilities and associated eigenfrequencies of the particular flow. Specifically, this method also requires that the flow be unstable to a range of disturbances, a condition which is not always satisfied.
Clearly, more efficient control of mixing in fully turbulent shear flows might be achieved by direct (rather than hierarchical) control of both the large-scale entrainment and the small-scale mixing processes. Such a control method has, before now, not been available but is enabled by synthetic jet actuators that are the subject of the present disclosure.
Some common applications of mixing in a bounded volume are mixing in chemical lasers and mixing for chemical or pharmaceutical products. In addition to these fields, the development of methods for enhancement of mixing through manipulation of the flow in which it occurs will have a direct impact on the performance of various other technologically important systems (e.g., in bioengineering).
Cooling of Heated Bodies
Cooling of heat-producing bodies is a concern in many different technologies. Particularly, a major challenge in the design and packaging of state-of-the-art integrated circuits in single- and multi-chip modules (MCMs) is the ever increasing demand for high power density heat dissipation. While current technologies that rely on global forced air cooling can dissipate about 4 W/cm2, the projected industrial cooling requirements are 10 to 40 W/cm2 and higher within the next five to ten years. Furthermore, current cooling technologies for applications involving high heat flux densities are often complicated, bulky and costly.
Traditionally, this need has been met by using forced convective cooling using fans which provide global overall cooling when what is often required is pinpoint cooling of a particular component or set of components. Furthermore, magnetic-motor-based fans have the problem of generating electromagnetic interference which can introduce noise into the system.
In applications when there is a heat-producing body in a bounded volume, the problem of cooling the body is substantial. In fact, effective cooling of heated bodies in closed volumes has also been a long standing problem for many designers. Generally, cooling by natural convection is the only method available since forced convection would require some net mass injection into the system, and subsequent collection of this mass. The only means of assistance would be some mechanical fan wholly internal to the volume. However, often this requires large moving parts in order to have any success in cooling the heated body. These large moving parts naturally require high power inputs. But, simply allowing natural convective cooling to carry heat from the body producing it into 4he fluid of the volume and then depending on the housing walls to absorb the heat and emit it outside the volume is a poor means of cooling.
Briefly described, the present invention involves synthetic jet actuators designed to exploit the inherent advantages of micromachining technology. Various novel applications of micromachined synthetic jet actuators, or microjets, are also included in the present invention.
A first object of the present invention is to provide an improved device for asserting indirect, non-intrusive control over a fluid flow. Most of the previous approaches to flow control can be classified as direct contact actuators. That is, prior art actuators generally have mechanical moving parts that come into direct contact with the flow in order to effect control authority. In contrast to these approaches, the fluidic technology based on synthetic jet actuators, which is the subject of the present invention, allows indirect assertion of control authority.
Another object of the present invention is for producing a synthetic jet stream of fluid synthesized from the working fluid of the medium where the synthetic jet actuator is deployed. Thus, linear momentum is transferred to the flow system without net mass injection into the system.
Another object of the present invention is to provide a synthetic jet actuator producing a fluidic jet stream for actively controlling fluid flows while eliminating the need for any complex piping or plumbing to supply fluid to the jet actuator.
Another object of the present invention is to provide a jet actuator which responds very quickly to control inputs and is able to operate effectively at high frequencies.
Another object of the present invention is for the production of synthetic jet actuators to control fluid flow fields with micromachining techniques in order to capitalize on the inherent advantages to micromachining manufacturing techniques.
Another object of the present invention is for the use of micromachined zero net mass flux synthetic jet actuators to create fluid flow in a bounded, or even sealed, volume for various cooling and/or mixing applications.
I. Construction and Operation of Synthetic Jet Actuators
The construction and operation of a basic, macro-scale synthetic jet actuator will first be described below. This actuator serves as the basis for the present invention and will aid in understanding the physics behind the micromachined synthetic jet actuators of the present invention. Full size synthetic jet actuators are described in detail in prior-filed patent application Ser. No. 08/489,490, filed Jun. 12, 1995, which is incorporated fully herein by reference. After discussing full-scale synthetic jet actuators, micromachined synthetic jet actuators will be briefly described and a preferred application will also be briefly discussed.
A. Basic Construction of Synthetic Jet Actuators
Although there are several possible configurations for a synthetic jet actuator, the most simple will be briefly described as an example. A basic macro-scale synthetic jet actuator preferably comprises a housing defining an internal chamber. An orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within said internal chamber so that a series of fluid vortices are generated and projected in an external environment out from the orifice of the housing. The volume changing mechanism can be any suitable mechanism, for instance, a piston positioned in the jet housing to move so that fluid is moved in and out of the orifice during reciprocation of the piston. Preferably, the volume changing mechanism is implemented by using a flexible diaphragm as a wall of the housing. The flexible diaphragm may be actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is utilized to create time-harmonic motion of the diaphragm. As the diaphragm moves into the chamber, decreasing the chamber volume, fluid is ejected from the chamber through the orifice. As the fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates vortex sheets which roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity.
As the diaphragm moves outward with respect to the chamber, increasing the chamber volume, ambient fluid is drawn from large distances from the orifice into the chamber. Since the vortices are already removed from the edges of the orifice, they are not affected by the ambient fluid being entrained into the chamber. As the vortices travel away from the orifice, they synthesize a jet of fluid, a xe2x80x9csynthetic jet,xe2x80x9d through entrainment of the ambient fluid.
B. Micromachining Synthetic Jet Actuators
The present invention involves use of micromachining techniques in the development of novel synthetic jet actuators for various fluid flow control applications. Micromachining is traditionally defined as the use of microfabrication technologies to create mechanical structures, potentially in addition to electronic devices. The use of microfabrication technologies gives to actuators the same advantages which integrated circuits enjoy, namely batch fabrication, and ease of realization and interconnection of large, cooperative actuator arrays. Another advantage may be small size. However, it is not required in the present invention that small size be maintained. As such, xe2x80x9cmicromachiningxe2x80x9d synthetic jet actuators should properly be defined as the use of batch fabrication technologies in a broad sense, without limiting it to integrated circuit fabrication technologies or other very small scale actuators.
1. Basic Micromachined Actuator Design
Micromachined synthetic jet actuators, or xe2x80x9cmicrojetxe2x80x9d actuators, are preferably fabricated from a substrate defining an actuator cavity with an orifice permitting fluid communication between the cavity and an external environment. Preferably, the actuator cavity is bounded at least partially by a flexible membrane. Vibration of the membrane preferably using either an electrostatic or piezoelectric drive results in a turbulent air jet formed approximately normal to the microjet orifice. As in larger-scale geometries, the synthetic jet stream is synthesized by a train of vortex rings. Each vortex is formed by the motion of the diaphragm and is advanced away from the microjet orifice under a self-induced velocity. The vortices are formed at the excitation frequency of the membrane and the jet stream of fluid is synthesized by vortex interaction with ambient fluid downstream from the orifice. The excitation frequency may vary widely depending on the application in which the microjet is used.
Another preferred embodiment for a microjet actuator comprises simulating a xe2x80x9cpiston in cylinderxe2x80x9d arrangement to take the functional place of the vibrating diaphragm. This can be accomplished by changing the aspect ratio of the actuator cavity to a deeper, more cylindrical shape. A piston-like actuator may then be realized by using a xe2x80x9cbossedxe2x80x9d diaphragm. A bossed diaphragm, for example, may comprise a diaphragm in which a thick mass is fabricated on the center and protrudes into the cylindrical actuator cavity upon actuation of the diaphragm. In addition, corrugations can be incorporated in the supporting diaphragm in order to increase flexibility of the piston support.
2. Specific Construction of Single Microjet Actuators
A first preferred embodiment for a microjet employs traditional micromachining technologies to realize these microjets. Initially, a high resistivity silicon wafer is preferably employed as a substrate for the device. Next, a layer of silicon dioxide is preferably deposited on both a top and a bottom side of the silicon wafer. Wet thermal oxidation is the preferred method of applying the layer of silicon dioxide. A layer of aluminum is then deposited on the top side of said wafer and an orifice hole is patterned in the aluminum layer.
A matching orifice hole is also created on the bottom side of the wafer using a photolithography process. Next, a jet orifice is anisotropically etched on the top side of said wafer and an actuator hole is formed in the bottom side of the wafer. The actuator hole is formed by a photolithography process.
An actuator cavity is formed in the wafer by anisotropically etching the actuator hole to an increased depth. The wafer is then preferably re-oxidized using thermal oxidation such that a layer of silicon dioxide is formed in the actuator cavity. A layer of aluminum is then sputtered on the bottom side of the silicon wafer to act as a first electrode for an electrostatic actuation method.
A flexible membrane, preferably a layer of polyimide film, is then appropriately bonded to the bottom side of the wafer to form a flexible actuation diaphragm and this film is then coated with a layer of aluminum using DC sputtering. This last layer of aluminum comprises a second electrode for the electrostatic actuation of the polymide film membrane. When the film is actuated, the device functions as a synthetic jet actuator of very small scale. This device may also include a power source for actuation and a control system, such as a microcomputer.
An improvement to microjets is the use of modulators with the jet actuators. Modulators are generally devices to selectively control flow through the orifice of a synthetic jet actuator (such as by covering and uncovering the orifice) in order to prevent either flow into or out of the jet cavity. In essence, a modulator controls the functioning of the synthetic jet actuator.
Modulators usually are constructed in two forms: vertical drive and lateral drive modulators. Vertical drive modulators move in the direction of the jet flow in order to seal the orifice hole. The modulators typically are designed as a xe2x80x9cflapxe2x80x9d attached to the substrate inside the actuator cavity by a hinge-like mechanism. An electrical impulse controls the modulator motion to cover and uncover the orifice. Lateral drive modulators, on the other hand, can be thought of as xe2x80x9cshuttersxe2x80x9d which slide to partially or totally occlude the jet orifice hole. If required, standard overpressure stops may be used with either type of modulator to prevent damage to the modulators. A third type of modulator can be formed as an xe2x80x9cinflatable collarxe2x80x9d about the microjet orifice. When the collar is inflated, flow through the orifice is restricted, or even stopped. Of course, all of the above-described modulators may be formed using micromachining technologies. That is, the modulators may be batch fabricated along with the rest of the microjet. In this way, modulator control circuitry can also be fabricated.
3. Amplification of Microjets
Many applications of synthetic jet actuators may require an unusually small apparatus. While conventional jet actuators may not be practical on such a small scale, because synthetic jet actuators can draw so much of their power from another flow, and are so simple in design, they are ideal for a micro-scale embodiment. Even though small in size, if used near another free flow, a synthetic jet actuator will draw power from the other flow through entrainment of the other flow. On the other hand, if the synthetic jet actuator is in a bounded volume, several synthetic jet actuators may be arrayed to build upon one another into a type of xe2x80x9ccascadedxe2x80x9d control.
While there is no question that one of the most important application areas for microactuators is the control of macro-events, as noted above, these actuators usually generate insufficient force to directly realize control authority. Thus, some type of amplification is often required. An attractive means for the amplification of the actuator output is its coupling to inherently unstable pressure or flow systems. If system operating points are carefully chosen, the relatively small forces generated by a microactuator can be used to create large disturbances in either static, pressure-balanced systems or in free and wall-bounded shear flows. For example, in the area of jet thrust vectoring, millimeter-scale actuators can be used for thrust vectoring of larger jets having characteristic length scales that are at least two orders of magnitude larger. The energy necessary for manipulation is extracted from the vectored flow and thus the power input to the actuator is of the order of several milliwatts. Use of millimeter-scale microjets to control larger jets suggests the concept of cascaded control. Namely, that microjet actuators be used to manipulate millimeter-scale jets which, in turn, will control larger jets.
4. Arrays of Microjets
Some applications of flow control may best be suited for whole arrays of microjet actuators. Arrays of microjet actuators are particularly attractive for applications such as jet vectoring because they can be individually addressed and phased. The first preferred embodiment of an addressable microjet actuator array comprises an array of small orifices situated on top of an array of actuator cavities. Both the orifices and the cavities are batch fabricated from  less than 100 greater than  silicon using micromachining techniques. Alternatively, silicon may be replaced with other cheaper or more robust substrates.
In this first preferred embodiment, individual jet control can be achieved by use of a metallized flexible polyimide diaphragm. The metal electrodes on the diaphragm are patterned so that voltage can be individually applied to the region over each actuator cavity. A key feature of this design is that the diaphragm can be vibrated using either a commercial piezoelectric transducer to drive all array elements in parallel or a sinusoidal drive voltage applied to the flexible diaphragm of individual array elements. Driving the membrane in either fashion results in cavity pressure variations and a jet flow through the orifices. An individual jet is modulated by either reducing the amplitude of the drive voltage of an individual array element (for electrostatic drive) or by electrostatically modulating the diaphragm vibration amplitude for that element (for piezoelectric drive).
A second preferred embodiment for a microjet array comprises a housing defining a cavity. The volume of the cavity is altered by a volume changing means. Preferably, the volume changing means comprises a piezoelectrically driven membrane or a piston element. However, the volume changing means may comprise an electromechanical or magnetically driven flexible membrane, as well as a combustion force.
The housing of the second preferred embodiment comprises multiple orifices in a wall of the housing. These orifices preferably have individually controllable louvers or modulators adjacent to the orifices. In that way, flow through the orifices may be controlled individually by the louvers or modulators.
The volume changing means preferably periodically changes the volume of the cavity. As the volume is increased, fluid is drawn through the orifices and into the volume. As the volume is subsequently decreased, the fluid is forced out through the orifices that are open (i.e. not obstructed by modulators or louvers), forming vortex sheets at the orifices, which roll up into vortices. These vortices move away from the orifices and entrain an ambient fluid into a synthetic jet stream. As such, the second preferred embodiment produces an array of synthetic jet streams.
II. Features of and Applications for Micromachined Synthetic Jet Actuators
Generally, xe2x80x9cmicrojetsxe2x80x9d can accomplish the same tasks, in the same applications as conventional, larger, synthetic jet actuators. Synthetic jet actuators, whether conventional actuators or microjets, may be used to create fluid flow in a bounded, or sealed, volume. Synthetic jet actuators are also excellent for vectoring other fluid flows without mechanically intruding into the flow. Furthermore, synthetic jet actuators will enhance mixing of fluids through direct control of small scale mixing and will enhance cooling of various heat-producing bodies. All these applications are described in the parent application Ser. No. 08/489,490. The inherent characteristics of synthetic jets may be exploited by using microjets in several preferred applications.
A first preferred application for microjets is in the modification of a fluid flow about a body or surface. When a microjet or microjet array is embedded in a body in a fluid flow field, the actuators can alter the apparent aerodynamic shape of the body with respect to the flow field. For such an application, one or more microjets are preferably embedded in a solid body, or surface, with microjet orifices built into the body/surface. The interaction of the fluid flow about the body with a synthetic jet stream produced by the actuators will change the overall fluid flow field around the solid body.
In contrast to the prior art, a unique feature of microjet actuators is that they can effectively modify wall-bounded shear flows by creating closed recirculating flow regimes near solid surfaces. In fact, the synthetic jet stream actually penetrates the flow boundary layer to affect the overall flow field about the solid body. This clearly distinguishes the present invention from prior art attempts to merely energize the boundary layer. Additionally, closed flow regimes can be formed by microjet actuators when the actuators are placed in an embedding flow, because the jet actuators do not dispense new fluid into the flow. Thus, when microjet actuators are placed near solid surfaces in wall bounded flow, they result in a change in the apparent aerodynamic shape of the surface; hence they can be exploited for modification of aerodynamic performance measures such as lift or drag.
The first preferred application for the present invention is use of one or more microjet actuators to modify the aerodynamic shape of a lifting surface in a flow field. Such a lifting surface will typically comprise a wing or rotor blade. However, any other aerodynamic surface may benefit from the present preferred embodiment. A microjet actuator, or several actuators, embedded in a solid lifting surface creates a permanent recirculation region near each jet orifice. Such a recirculation region modifies both the flow field and pressure distribution around the aerodynamic surface thereby modifying both lift and drag characteristics of the surface. Preferably, an entire array of individually addressable microjets are distributed across the surface of the wingxe2x80x94either an upper or lower surface.
In particular, because the aerodynamic characteristics of an airfoil depend critically on the location of its front and rear stagnation points and on its camber and thickness, these characteristics can be altered by microjet actuators without the use of movable flaps. Placement of microjet arrays and the creation of closed recirculating flow regions along the leading and trailing edges and along the upper and lower surfaces of an airfoil can result in displacement of the airfoil""s front and rear stagnation points and change its apparent thickness and camber. Addressable microjet arrays can also be used to dynamically tailor and optimize aerodynamic performance preventing premature flow separation and thus loss of lift.
This application of the present invention will improve the efficiency of aircraft at various flight regimes. Typically, an aircraft wing is designed for optimum performance at a primary mission flight regime. As a consequence, the efficiency of the aircraft suffers in other regimes. However, with the present invention, microjet arrays can be tailored to alter the aerodynamic shape of the wing during inefficient flight regimes in order to dramatically improve overall aircraft efficiency.
It is important that microjet actuators are synthesized from the working fluid of the flow system in which they are employed. This feature obviates the need for expensive and bulky input piping and complex fluidic packaging. These attributes of zero net mass flux microjet actuators also make them ideally suited for low-cost batch fabrication. This is in stark contrast to the prior art usage of flaps or slats to change the flow about an aerodynamic surface.
Other applications for microjets will be obvious to one with skill in the art. For example, cooling electronic components with microjets is an important use of microjet actuators. Cooling applications are described in detail for conventional synthetic jet actuators in a concurrently filed Application entitled xe2x80x9cSynthetic Jet Actuators for Cooling Heated Bodies and Environmentsxe2x80x9d (filed on Nov. 13, 1997, with Ser. No. 08/970,607, now U.S. Pat. No. 6,123,145, which is incorporated herein by reference. Microjets, or microjet arrays, will also cool other heated bodies effectively. Such applications are intended to be included in the present invention.
An advantage to the present invention is that the synthetic jet actuators described above eliminate the need for complex piping or plumbing to carry fluid to the microjet actuator for any application in which it is used.
Another advantage to the present invention is that the zero net mass flux microjet actuator will be much easier to manufacture and be much more readily installed in places where space constraints are critical than other jet actuators.
Another advantage to the use of synthetic control jets for vectoring a fluid flow is that the two jets can be used in parallel. This simplifies the installation and maintenance of the primary jet and the synthetic control microjet.
Another advantage to the use of a microjet actuator to vector another primary jet is that microjet actuators vector economically. Because the energy necessary for the manipulation of the primary jet is extracted from the mean flow of the primary jet, the power input to the synthetic control jet can be quite small.
An additional advantage is that the controller bandwidth may be very high due to the fact that the primary flow responds at the frequency of the diaphragm of the microjet actuator. This driving frequency can vary from several hundred hertz to several kilohertz.
An additional advantage to the preferred embodiments of the present invention becomes obvious when a microjet actuator is used to control another microjet actuator. In this way, the direction of the synthetic jet pair, or array, can be dynamically and accurately controlled by merely modifying the phase angle of the two volume changing means in the jet actuator pair or by controlling the amplitude and phase of modulators fixed about the actuator orifices in an appropriate fashion. Complex and disruptive mechanical flow modifiers, which are necessary to modify the direction in the prior art, are not necessary to the present invention.
An additional advantage to microfabricated synthetic jet actuators is that microactuators typically consume small amounts of power, and they can be integrated onto a chip with microsensors and control electronics.
Another advantage of the present invention is that microjets may be realized by high density batch fabrication for lower cost and improved reliability.
Other features and advantages will become apparent to one with skill in the art upon examination of the following drawings and detailed description. All such additional features and advantages are intended to be included herein within the scope of the present invention, as is defined by the claims.